The term “polymer composite” broadly refers to polymer based compositions that are formulated to contain additives to improve their properties for a specific application. The polymer composites may contain, for example, reinforcing fibers, fillers, pigments and other polymers. Polymer composite components are available for use in many commercial applications. They offer great potential as relatively low weight body panels and other components in automotive vehicles.
In the case of automotive vehicle body panel applications, polymer composites include, for example, compression molded sheet molding compound (SMC) containing unsaturated polyester and polystyrene resins, reinforced reaction injection molded (RRIM) polyureas, or injection molded products containing poly (phenylene oxide) (PPO)/nylon based resins. Such polymer composite parts are lighter than comparably sized steel panels. However, the composites do have to be painted for body panel applications, and it has been difficult to paint the composite body panels without introducing surface defects.
Automotive painting operations are typically carried out on a body-in-white. A body-in-white is the unpainted unitary body structure comprising welded, or otherwise attached, body panels and structural components. Such a body structure is usually formed mostly of steel panels but now may include some polymer composite panels. The paint shop practice is established for the steel portion of the body which is electrically conductive and receives several coating layers for corrosion resistance, paint adhesion and painted surface finish quality. The composite panels do not respond to the several coating procedures in the same way as the steel panels. For example, automotive painting operations often involve the separate application of a zinc phosphate layer, an electrocoated liquid (i.e., using water or an organic solvent) prime, a liquid or powder primer surfacer layer, a liquid base color coat and a liquid or powder clear top coat. But there is no deposition of either the zinc phosphate coating or the electrocoated prime on the typical composite panel surfaces.
Following each of the prime coat, the primer surfacer and the clear top coat applications there is a baking step at temperatures of 250° F. or higher to cure or dry the new layer and to promote flow of the top coat films to a commercially acceptable finish for a vehicle. Such aggressive heating of the painted composites typically leads to “out-gassing.” Out-gassing is the release of entrapped air, solvent, moisture, and uncured chemicals and polymer precursor materials from the somewhat porous composite substrate. The result too often is an unsightly and unacceptable rough surface. Out-gassing was initially experienced with liquid primer surfacer paints at their 250° F. bake temperature. The occurrence of surface roughness with such paint systems has been reduced in some instances by the use of a special formulated, electrically conductive polymer prime coat as a barrier coat after molding. This polymeric prime coat on the composite surface may reduce out-gassing at that location. But this coating doesn't appear to work for all molded polymer composite and liquid paint combinations, and it completely fails to prevent out-gassing during the flow and curing of powder paints which require even higher bake temperatures (350° F.).
Accordingly, it is an object of the invention to provide a method of treating the surfaces of polymer composite articles of manufacture to avoid out-gassing caused defects during post-molding painting operations. More specifically, it is an object of this invention to provide a conductive metal coating on molded polymer composite surfaces to permit, for example, the phosphating and subsequent prime coatings and top coatings of automotive body panels yielding uniform appearing and high quality surface finishes.